Fire retardant gypsum panels and other products of many useful shapes can be produced by dehydrating and rehydrating gypsum, also known as calcium sulfate dihydrate or landplaster. Such gypsum products are commonly used in building construction.
As gypsum by itself is not water-resistant, there has been a considerable amount of work on improving water resistance of gypsum products. Hydrocarbons, including wax and asphalt are suggested to reduce water uptake in U.S. Pat. No. 2,198,776. Materials including metallic soaps and silicones have been used as coatings on the surface of a finished gypsum product. However, it is difficult to obtain a coating made of these materials with consistent water resistance.
U.S. Pat. No. 4,411,701 discloses adding alkaline metal alkylsiliconates or phenylsiliconates together with calcium hydroxide or calcium oxide to gypsum in making waterproof gypsum molded products, while U.S. Pat. No. 4,371,399 discloses a water-repellent gypsum mortar comprising certain fatty amines. U.S. Pat. No. 7,294,195 discloses water repellant gypsum compositions comprising a hydroxyl-based cellulose and a siliconate.
Siloxane emulsions can be added to a gypsum slurry and these emulsions are useful for improving water-resistance of a gypsum product. A siloxane emulsion was added to fiberboard gypsum products in U.S. Pat. No. 5,817,262 to Englert. In addition to siloxane emulsions, siloxane dispersions can also be used for improving water-resistance of a gypsum product, as described in U.S. Pat. No. 7,413,603 to Miller.
Adding siloxane to a gypsum slurry improves significantly water resistance of a resulting gypsum product. However, a gypsum product is exposed to high temperatures while drying in a kiln and during this high temperature exposure some of the siloxane inadvertently evaporates from the gypsum product and is converted to silicon oxide, also known as siloxane dust. As siloxane dust accumulates in a kiln, it requires stopping production from time to time to clean the kiln and remove the siloxane dust. A kiln has to be cooled down before it can be cleaned and then reheated again before production of gypsum products can be resumed again. These repetitive cooling/heating cycles lead to losses in fuel and waste of energy. Additionally, the siloxane dust can accumulate on gas burners, hot air nozzles, and other equipment within the kiln system, causing the equipment to operate less efficiently and increasing further fuel costs. The siloxane dust can also accumulate on fans, causing them to become unbalanced, resulting in premature bearing failure and high maintenance costs.